Extensional Stress-Induced Orientation and Crystallization can Regulate the Balance of Toughness and Stiffness of Polylactide Films: Interplay of Oriented Amorphous Chains and Crystallites

Melt stretching and quenching produce low-crystalline biodegradable poly(lactic acid) filled with β-form shish for highly improved mechanical toughness

High-toughness biodegradable poly(lactic acid) (PLA) has always been intensively pursued on the way of replacing traditional petroleum-based plastics. Regulating microstructures to achieve self-toughening holds great promise due to avoidance of incorporating other heterogeneous components. Herein, we propose a straightforward and effective way to tailor microstructures and properties of PLA through melt-stretching and quenching of slightly crosslinked samples. The melt stretching drives chains orientation and crystallization at high temperature, while the quenching followed can freeze the crystallization process to any stage. For the first time, we prepare a type of transparent and low-crystalline PLA filled with rod-like β-form shish, which displays an outstanding tensile toughness, almost 17 times that of the conventional technique-processed one. This mechanical superiority is enabled by an integration of high ductility due to oriented chain network, and high tensile stress endowed by nanofibrous filler's role of β-form shish. Furthermore, the mechanically toughened PLA is demonstrated to generate the richest micro-cracks and shear bands under loading, which can effectively dissipate the deformational energy and underlie the high toughness. This work opens a new prospect for the bottom-up design of high-performance bio-based PLA materials that are tough, ductile and transparent by precise microstructural regulation through scalable melt processing route.

Degradation of Structurally Modified Polylactide under the Controlled Composting of Food Waste

The degradation of polylactide (PLA) films of different structures under conditions of controlled composting has been studied. We have demonstrated that PLA underwent degradation within one month in a substrate that simulated standard industrial composting. Regardless of the initial structure of the samples, the number-average molecular weight (Mn) decreased to 4 kDa while the degree of crystallinity increased to about 70% after 21 days of composting. Addition of an inoculant to the standard substrate resulted in the accelerated degradation of the PLA samples for one week due to an abiotic hydrolysis. These findings have confirmed that industrial composting could solve the problem of plastic disposal at least for PLA.

Robust Osteoconductive β-Tricalcium Phosphate/L-poly(lactic acid) Membrane via Orientation-Strengthening Technology

L-poly(lactic acid) (PLLA) is a biodegradable material with multiple biomedical application potentials, especially as a membrane for guided bone regeneration. In terms of its low strength and poor osteogenic activity, improving these two properties is the key to resolve the limitations of PLLA for bone-associated applications. Herein, an orientation-strengthening technology (OST) was developed to reinforce PLLA's mechanical strength by introducing biocompatible β-tricalcium phosphate (β-TCP) to improve the crystallinity of PLLA, allowing for the formation of a highly oriented architecture to acquire an advanced membrane with high mechanical property. Furthermore, the addition of β-TCP nanoparticles significantly promotes the osteogenic activity of the composites. The tensile strength of the membrane containing 5 wt % β-TCP was 220 MPa, which was 4-folds that of the native polylactic acid fabricated via the conventional method. The oriented microstructure enhanced both the mechanical strength and the osteogenic activity of the material. The parallel grooves on the material surface are similar to the mineralized collagen fibers on the bone surface, which promoted the growth and differentiation of osteoblasts, with β-TCP further contributing to the osteoconductive effect. The combination of β-TCP and orientation-strengthening effect endows the material with higher mechanical properties and bioactivities, which provides an advanced manufacturing strategy for the preparation of PLLA-based materials for bone repair.

A Versatile Disorder-to-Order Technology to Upgrade Polymers into High-Performance Bioinspired Materials

Biodegradable polymer as traditional material has been widely used in the medical and tissue engineering fields, but there is a great limitation as to its inferior mechanical performance for repairing load-bearing tissues. Thus, it is highly desirable to develop a novel technology to fabricate high-performance biodegradable polymers. Herein, inspired by the bone's superstructure, a versatile disorder-to-order technology (VDOT) is proposed to manufacture a high-strength and high-elastic modulus stereo-composite self-reinforced polymer fiber. The mean tensile strength (336.1 MPa) and elastic modulus (4.1 GPa) of the self-reinforced polylactic acid (PLA) fiber are 5.2 and 2.1 times their counterparts of the traditional PLA fiber prepared by the existing spinning method. Moreover, the polymer fibers have the best ability of strength retention during degradation. Interestingly, the fiber tensile strength is even higher than those of bone (200 MPa) and some medical metals (e.g., Al and Mg). Based on all-polymeric raw materials, the VDOT endows bioinspired polymers with improved strength, elastic modulus, and degradation-controlled mechanical maintenance, making it a versatile update technology for the massive industrial production of high-performance biomedical polymers.

Robust, Fully Biodegradable Films of Polyesters Realized by In Situ Formation of an Interconnected Multi-Nanolayer Structure under Extensional Flow

Multilayer structures are not only applied to manipulate properties of synthetic polymer materials such as rainbow films and barrier films but also widely discovered in natural materials like nacre. In this work, in situ formation of an interconnected multi-nanolayer (IMN) structure in poly(butylene adipate-co-terephthalate) (PBAT)/poly(butylene succinate) (PBS) cocontinuous blends is designed by an extensional flow field during a "casting-thermal stretching" process, combining the properties of two components to a large extent. Hierarchical structures including phase morphology, crystal structure, and lamellar crystals in IMN films have been revealed, which clearly identifies the crucial role of extensional flow. The oriented PBAT phase in the IMN structure can be beneficial to the epitaxial growth of PBS crystals onto the PBAT nanolayers, thus improving interfacial adhesions. Furthermore, intense extensional stress can also promote crystallinity and thicken the lamellar structure. Given such distinct features in the fully biodegradable films, a simultaneous enhancement in tear strength, tensile strength, and puncture resistance has been achieved. To the best of our knowledge, the tear strength of IMN films about 285.9 kN/m is the highest level in the previous works of this system. Moreover, the proposed fabrication way of the IMN structure is facile and scalable, which is highly expected to be an efficient strategy for development of structured biodegradable polymers with excellent comprehensive properties.

Highly Enhanced Mechanical, Thermal, and Crystallization Performance of PLA/PBS Composite by Glass Fiber Coupling Agent Modification

To improve the toughness and heat resistance of polylactic acid (PLA), polybutylene succinate (PBS) was sufficiently blended with PLA as the base matrix, and the glass fiber (GF) that was modified with 3-aminopropyltriethoxysilane (KF-GF) was added as the reinforcement. The results demonstrated a noteworthy boost in both mechanical and heat resistance properties when employing KH-GF, in comparison to pristine GF. When the content of KH-GF reached 20%, the tensile, flexural, and IZOD impact strength of the composites were 65.53 MPa, 83.43 MPa, and 7.45 kJ/m2, respectively, which were improved by 123%, 107%, and 189% compared to the base matrix, respectively. This enhancement was primarily attributed to the stronger interfacial adhesion between KH-GF and the PLA/PBS matrix. Furthermore, the Vicat softening temperature of the composites reached 128.7 °C, which was a result of increased crystallinity. In summary, the incorporation of KH-GF into PLA/PBS composites resulted in notable enhancements in their mechanical properties, crystallinity, and thermal characteristics. The high performance KH-GF-reinforced PLA/PBS composite showed a broad application potential in the field of biodegradable packaging, biodegradable textiles, and biodegradable plastic bags.

Extensional Stress-Induced Ductility of Poly(l-lactide) Films: Role of the Entangled Network in Amorphous Regions

The relationship between the density of the entangled amorphous network and the ductility of oriented poly(l-lactide) (PLLA) films is explored based on the preferential hydrolysis of the amorphous regions in phosphate buffer solution (PBS). PLLA films with a balance of ductility and stiffness have been prepared by the "casting-annealing stretching" based on mechanical rejuvenation, and the structural evolution and mechanical properties at different hydrolysis durations have been identified. Various stages are found during the transition of ductility to brittleness for hydrolyzed PLLA films. First, the elongation at break for hydrolyzed PLLA films remains unchanged in the first stage of hydrolysis and then gradually decreases. Eventually, the films turn to be brittle in the third stage. The strain-hardening modulus (G_R) of the hydrolyzed films is utilized to reflect the density of the entangled amorphous network, and a gradual decrease of G_R with hydrolysis time indicates the decisive role of the amorphous entanglement network in the mechanical rejuvenation-induced ductility of PLLA. The quantitative relationship between the entangled amorphous network and the stress-induced ductility of PLLA films is revealed. The dependence of deformation behavior on entangled amorphous network density is closely correlated to activated primary structure during deformation. The intact chain network plays a crucial role in sufficiently activating the primary structure to yield and disentanglement during the subsequent necking. These findings could advance the understanding of the PLLA's ductility induced by mechanical rejuvenation and offer guidance for awakening the intrinsic toughness of PLLA.

Enhanced crystallization, heat resistance and transparency of poly(lactic acid) with self-assembling bis-amide nucleator

The application of poly(lactic acid) (PLA) is limited by its low crystallization rate. Conventional methods to increase crystallization rate usually result in a significant loss of transparency. In this work, a bundled bis-amide organic compound N'-(3-(hydrazinyloxy)benzoyl)-1-naphthohydrazide (HBNA) was used as a nucleator to prepare PLA/HBNA blends with enhanced crystallization, heat resistance and transparency. HBNA dissolves in PLA matrix at high temperature and self-assembles into bundle microcrystals by intermolecular hydrogen bonding at a lower temperature, which induces PLA to form ample spherulites and "shish-kebab-like" structure rapidly. The effects of HBNA assembling behavior and nucleation activity on the PLA properties and the corresponding mechanism are systematically investigated. As a result, the crystallization temperature of PLA increased from 90 °C to 123 °C by adding as low as 0.75 wt% of HBNA, and the half-crystallization time (t_1/2) at 135 °C decreased from 31.0 min to 1.5 min. More importantly, the PLA/HBNA maintains good transparency (transmittance > 75 % and haze is ca. 27 %) due to the decreased crystal size, even though the crystallinity of PLA is increased to 40 %, which also led to good heat resistance. The present work is expected to expand the application of PLA in packaging and other fields.

Role of Shear Flow on Structure Development during Post-Processing Annealing for Poly(lactic acid)

The effect of shear history on structure development during post-processing annealing was studied using poly(lactic acid) PLA. Since PLA shows a low crystallization rate, quenched films had no crystallinity. Moreover, molecular orientation was not detected in the films. During the annealing procedure beyond its glass transition temperature, however, molecular orientation to the flow direction occurred with the crystallization growth in the films having an appropriate shear history. This peculiar crystal growth during the annealing was most probably attributed to the crystallization from extended chain crystals generated during the applied shear history, although the amount of extended chain crystals was low. The results obtained in this study should be noted because the molecular orientation proceeded due to the annealing history applied. Furthermore, this phenomenon will be used to suppress dimensional change and increase product rigidity.

Revisiting the Contribution of Additives to the Long-Term Mechanical Stability and Hydrolytic Resistance of Highly Crystalline Polylactide Fibers

Additives are widely used to improve the processability, toughness, and hydrolytic resistance of poly(lactic acid) (PLA)-based materials. This study compares neat PLA fibers and fibers made from PLA blends with either poly(butylene succinate) (PBS) as a plasticizer or poly(d-lactic acid) (PDLA) as a nucleating agent. The fibers have been characterized with regard to their physical and structural properties after fabrication as well as after artificial aging at elevated temperature and humidity conditions. All samples have been fabricated using industrial melt-spinning equipment, resulting in a high crystallinity of about X_C = 80% and a good initial toughness. Long-term relaxation behavior has been assessed with a self-developed lifetime prediction model, which is successfully verified for semicrystalline blended fibers. Despite slight improvement of the fiber elasticity and ductility, both types of blended fibers demonstrated a reduced hydrolytic resistance. These results suggest a design strategy for neat durable PLA fibers through processing-induced high crystallinity and orientation, which provide improved hydrolytic stability while preserving tough mechanical performance.

Morphology of Shear-Induced Polymer Cylindrites Revealed by 3D Optical Imaging

Two-photon confocal laser microscopy was used to obtain three-dimensional (3D) images of the morphology of poly(lactic acid) after shear-induced crystallization. The necessary fluorescence contrast was achieved by doping the polymer with Nile Red. The dye gets partially rejected from the growing crystalline aggregates during their formation, thus creating a renderable high-low fluorescence boundary outlining the shape of the aggregates. Parallel-plate melt-shearing and pulling a glass fiber through the melt were used as the two methods to achieve shear-induced crystallization. This study focuses on the shape of the resulting cylindrites, i.e., large-diameter shish-kebabs. The first 3D images of polymer cylindrites show that, if far from boundaries, they are circular cylinders, highly regular after fiber pull, but less so after parallel-plate shear. In the latter case, the cylindrite reveals the trajectory of the foreign particle that had nucleated its growth. Interestingly, lateral growth of the cylindrites was found to accelerate toward the sample surface when approaching it, giving the cylindrite an elliptical cross section. Furthermore and surprisingly, in the case of fiber pull, a row of spherulites is nucleated at the polymer-substrate interface nearest to the fiber, aligned along the fiber axis and appearing ahead of the rest of the space-filling spherulites. Both the phenomena, elliptical cylindrites and row of spherulites, are attributed to negative pressure buildup peaking at the cylindrite growth front and at the nearby film surface caused by crystallization-induced volume contraction. The pressure and flow distribution in the system is confirmed by numerical simulation. The results illustrate the value of 3D imaging of crystalline morphology in polymer science and polymer processing industry.

Key Factors of Mechanical Strength and Toughness in Oriented Poly(l-lactic acid) Monofilaments for a Bioresorbable Self-Expanding Stent

The investigation of the strength and toughness of poly(l-lactic acid) (PLLA) monofilaments is essential as the fundamental element of a biodegradable braided stent. However, the determining factor remains poorly addressed with respect to influencing the mechanical behavior of PLLA monofilaments. In this work, the electron beam (EB) with different radiation doses was utilized to sterilize PLLA monofilaments. Properties of the monofilaments, including the breaking strength, elongation at break, molecular weight, orientation, and microstructure of the fracture, were characterized. Results showed that a random chain scission of PLLA resulting from EB during this process could cause the decrease in molecular weight, which led to the decline in breaking strength. Meanwhile, the irradiated monofilaments were found to have almost the same elongation at break below a dose of 30 kGy and declined by 71.41% up to a dose of 48 kGy. It was also found that the ductile fracture connection of the monofilament translated to the brittle fracture by comparing the microstructure without and with sterilization. These phenomena could originate from the destruction of the long molecular chains connecting the crystal plates into shorter ones by radiation. PLLA monofilaments with 0, 30, and 48 kGy were used to braid carotid stents. Compared with a carotid Wallstent, the PLLA stent can better provide radial supporting to the carotid lesion. This study provides preliminary experimental references to evaluate and predict the mechanical performance of PLLA braided stents.

Reorganization of Hydrogen Bonding in Biobased Polyamide 5,13 under the Thermo-Mechanical Field: Hierarchical Microstructure Evolution and Achieving Excellent Mechanical Performance

The hierarchical microstructure evolution of an emerging biobased odd-odd polyamide 5,13 (PA5,13) films under the thermo-mechanical field, stepping from hydrogen bond (H-bond) arrangement to the crystalline morphology, has been investigated systematically. It is found that the reorganization of H-bonds under the thermo-mechanical field plays a crucial role in the crystallization of PA5,13. Especially, it is revealed that the crystallization process under the thermo-mechanical field develops along the chain axis direction, while lamellar fragmentation occurs perpendicular to the chain axis. Consequently, a stable and well-organized H-bond arrangement and lengthened lamellae with significant orientation have been constructed. Laudably, an impressive tensile strength of about 500 MPa and modulus of about 4.7 GPa are thus achieved. The present study could provide important guidance for the industrial-scale manufacture of high-performance biobased odd-odd PAs with long polymethylene segment in the dicarboxylic unit combined with a large difference between the polymethylene segments in the dicarboxylic and diamine units.

In-situ constructing highly oriented ductile poly(3-hydroxybutyrate-co-3-hydroxyvalerate) nanoribbons: Towards strong, ductile, and good heat-resistant polylactic-based composites

It remains a great challenge to manufacture polylactic (PLA) with high strength, ductility, and heat resistance simultaneously. Herein, PLA/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanoribboned composites, the highly oriented PHBV nanoribbons decorated by the PLA lamella, are successfully achieved through the multistage stretching extrusion (MSE) system. SEM confirms that in-situ highly oriented PHBV nanoribbons are achieved by biaxial-stretching field during the MSE process. Through investigating crystalline architecture of PLA/PHBV nanoribboned composites, it is found that the stiff shish and sparse lamellae of PLA are obtained under the coupling effect of PHBV nanoribbons and biaxial-stretching field. DMA reveals partial compatibility between PLA and PHBV. Interestingly, during tensile test, PHBV nanoribbons show high flexibility and synergistically facilitate the stretch of semi-rigid chains of PLA by an effective interfacial interaction. Consequently, even they both are extremely brittle, PLA/PHBV nanoribboned composites exhibit excellent strength (82.9 MPa) and ductility (186.7 %), compared with pure PLA (71.4 MPa and 12.3 %). Additionally, due to the promotion of the crystallization of PLA, PLA/PHBV nanoribboned composites show excellent heat resistance (E'_140°C > 350 MPa). The nanoribboned composites are of immense significance, which provide significant guidance for the simultaneous enhancement of ductility and strength of polymer materials.

Fabricating High-Thermal-Conductivity, High-Strength, and High-Toughness Polylactic Acid-Based Blend Composites via Constructing Multioriented Microstructures

The massive accumulation of plastic waste has caused a serious negative impact on the human living environment. Replacing traditional petroleum-based polymers with biobased and biodegradable poly(l-lactic acid) (PLLA) is considered an effective way to solve this problem. However, it is still a great challenge to manufacture PLLA-based composites with high thermal conductivity and excellent mechanical properties via tailoring the microstructures of the blend composites. In the present work, a melt extrusion-stretching method is utilized to fabricate biodegradable PLLA/poly(butylene adipate-co-butylene terephthalate)/carbon nanofiber (PLLA/PBAT/CNF) blend composites. It is found that the incorporation of the extensional flow field induces the formation of multioriented microstructures in the composites, including the oriented PLLA molecular chains, elongated PBAT dispersed phase, and oriented CNFs, which synergistically improve the thermal conductivity and mechanical properties of the blend composites. At a CNF content of 10 wt %, the in-plane thermal conductivity, tensile strength, and elongation at break of the blend composite reach 1.53 Wm^-1 K^-1, 66.8 MPa, and 56.5%, respectively, which increased by 31.9, 73.5, and 874.1% compared with those of the conventionally hot-compressed sample (1.16 Wm^-1 K^-1, 38.5 MPa, and 5.8%, respectively). The main mechanism for the improved thermal conductivity is that the multioriented structure promotes the formation of a CNF thermal conductive network in the composites. The strengthening mechanism is attributed to the orientation of both PLLA molecular chains and CNFs in the stretching direction, restricting the movement of PLLA molecular segments around CNFs, and the toughening mechanism is due to the transformation of PLLA molecular chains from low-energy gt conformers to high-energy gg conformers induced by extensional flow field. More interestingly, after the extrusion-stretched samples are annealed, the oriented PLLA molecular chains form oriented crystal structures such as extended-chain lamellae, common "Shish-kebabs," and hybrid Shish-kebabs, which further enhance the thermal conductivity and heat resistance of the samples. This work reveals the effects of the orientation of the matrix molecular chains and crystallites on the thermal conductivity and mechanical properties of composites and provides a new way to prepare high-performance PLLA-based composites with high thermal conductivity, excellent mechanical properties, and high heat resistance.

Poly (lactic acid) blends with excellent low temperature toughness: A comparative study on poly (lactic acid) blends with different toughening agents

Poly (lactic acid) (PLA) blends with different toughening agents were prepared by melt compounding, and the effects of toughening agents on the toughness of PLA, especially the low-temperature toughness, were investigated. All blends were immiscible systems, but the rheological Cole-Cole diagram showed that the blends had certain compatibility, and the interfacial bonding of PLA/Ethylene/butyl methacrylate/Glycidyl Methacrylate Terpolymer (GEBMA) blend was the best. With addition of the toughening agents, all blends showed improvement of the tensile and impact toughness both at room temperature and low temperature. GEBMA was the best toughening agent, the elongation at break and impact strength at room temperature and low temperature were greatly improved. The elongation at break, tensile strength and impact strength of PLA blend with 20 wt% GEBMA at -20 °C was 55.8 MPa, 195.9% and 18.8 kJ/m², respectively, which showed the reinforcement and super ductility at low temperature. However, the toughening effect of Poly (propylene carbonate) polyurethane (PPCU) at low temperature was poor. The T_g and interfacial bonding were the main factors affecting the toughness of the blends, especially at low temperature. The lower the T_g and the better the interfacial bonding, the better the toughness of the blends.

Deformation of Poly-l-lactid acid (PLLA) under Uniaxial Tension and Plane-Strain Compression

The ability of PLLA, either amorphous or semicrystalline, to plastic deformation to large strain was investigated in a wide temperature range (Td = 70-140 °C). Active deformation mechanisms have been identified and compared for two different deformation modes-uniaxial drawing and plane-strain compression. The initially amorphous PLLA was capable of significant deformation in both tension and plane-strain compression. In contrast, the samples of crystallized PLLA were found brittle in tensile, whereas they proved to be ductile and capable of high-strain deformation when deformed in plane-strain compression. The main deformation mechanism identified in amorphous PLLA was the orientation of chains due to plastic flow, followed by strain-induced crystallization occurring at the true strain above e = 0.5. The oriented chains in amorphous phase were then transformed into oriented mesophase and/or oriented crystals. An upper temperature limit for mesophase formation was found below Td = 90 °C. The amount of mesophase formed in this process did not exceed 5 wt.%. An additional mesophase fraction was generated at high strains from crystals damaged by severe deformation. After the formation of the crystalline phase, further deformation followed the mechanisms characteristic for the semicrystalline polymer. Interlamellar slip supported by crystallographic chain slip has been identified as the major deformation mechanism in semicrystalline PLLA. It was found that the contribution of crystallographic slip increased notably with the increase in the deformation temperature. The most probable active crystallographic slip systems were (010)[001], (100)[001] or (110)[001] slip systems operating along the chain direction. At high temperatures (Td = 115-140 °C), the α→β crystal transformation was additionally observed, leading to the formation of a small fraction of β crystals.

Orientation of Polylactic Acid-Chitin Nanocomposite Films via Combined Calendering and Uniaxial Drawing: Effect on Structure, Mechanical, and Thermal Properties

The orientation of polymer composites is one way to increase the mechanical properties of the material in a desired direction. In this study, the aim was to orient chitin nanocrystal (ChNC)-reinforced poly(lactic acid) (PLA) nanocomposites by combining two techniques: calendering and solid-state drawing. The effect of orientation on thermal properties, crystallinity, degree of orientation, mechanical properties and microstructure was studied. The orientation affected the thermal and structural behavior of the nanocomposites. The degree of crystallinity increased from 8% for the isotropic compression-molded films to 53% for the nanocomposites drawn with the highest draw ratio. The wide-angle X-ray scattering results confirmed an orientation factor of 0.9 for the solid-state drawn nanocomposites. The mechanical properties of the oriented nanocomposite films were significantly improved by the orientation, and the pre-orientation achieved by film calendering showed very positive effects on solid-state drawn nanocomposites: The highest mechanical properties were achieved for pre-oriented nanocomposites. The stiffness increased from 2.3 to 4 GPa, the strength from 37 to 170 MPa, the elongation at break from 3 to 75%, and the work of fracture from 1 to 96 MJ/m³. This study demonstrates that the pre-orientation has positive effect on the orientation of the nanocomposites structure and that it is an extremely efficient means to produce films with high strength and toughness.

Improvement of the strength and toughness of biodegradable polylactide/silica nanocomposites by uniaxial pre-stretching

Highly toughened polylactide (PLA) nanocomposites with balanced stiffness and strength were successfully prepared by combining the modification of 5 wt% silica (SiO₂) nanoparticles and uniaxial pre-stretching. The PLA/5 wt% SiO₂ nanocomposites fractured in a brittle way due to the network structure composed of cohesional entanglements. After pre-stretching, the elongation at break was increased to 168% at pre-stretching ratio (PSR) of only 0.5, which should be attributed to the destruction of the network structure of cohesional entanglements. With the increment of PSR, the modulus and tensile strength were improved obviously (2725 MPa, 101.6 MPa at PSR = 2.0) while the elongation at break (56% at PSR = 2.0) reduced gradually because of the formation of orientation and mesophase. However, the elongation at break was still larger than that of undrawn PLA (5.4%) and undrawn PLA nanocomposites (7.2%), indicating that the uniaxial pre-stretching was an effect way to strengthen and toughen PLA nanocomposites.

Preparation and properties of poly(L-lactic acid) blends with excellent low-temperature toughness by blending acrylic ester based impact resistance agent

Poly(L-lactic acid) (PLLA) blends with excellent low-temperature toughness and strength were prepared by melt compounding with acrylic ester based impact resistance agent (AEIR). The morphology, thermal properties, mechanical properties and biodegradability of the blends were investigated. Morphology observations revealed the blend was immiscible but had good compatibility with the dispersed phase size of about 200-300 nm. With the addition of AEIR, dramatic improvement in toughness of PLLA was achieved in a wide temperature range, especially at low temperatures the tensile strength was effectively remained. For the blend with 20 wt% AEIR, the tensile strength, elongation at break and impact strength were 51.6 MPa, 72% and 77.1 KJ/m² at -20 °C, respectively, much greater than that reported. The calculated T_g of AEIR was lower than the test temperatures, and the brittle-tough transition occurred. The PLLA matrix demonstrated obvious shear yielding which induced energy dissipation and therefore lead to excellent toughness of the blends. Moreover, the biodegradation of PLLA was enhanced after blends preparation.

Modification of Polylactide Nonwovens with Carbon Nanotubes and Ladder Poly(silsesquioxane)

Electrospun nonwovens of poly(L-lactide) (PLLA) modified with multiwall carbon nanotubes (MWCNT) and linear ladder-like poly(silsesquioxane) with methoxycarbonyl side groups (LPSQ-COOMe) were obtained. MWCNT and LPSQ-COOMe were added to the polymer solution before the electrospinning. In addition, nonwovens of PLLA grafted to modified MWCNT were electrospun. All modified nonwovens exhibited higher tensile strength than the neat PLA nonwoven. The addition of 10 wt.% of LPSQ-COOMe and 0.1 wt.% of MWCNT to PLLA increased the tensile strength of the nonwovens 2.4 times, improving also the elongation at the maximum stress.

Durably Ductile, Transparent Polystyrene Based on Extensional Stress-Induced Rejuvenation Stabilized by Styrene-Butadiene Block Copolymer Nanofibrils

The glassy polymer of polystyrene (PS) enjoys a good reputation as a promising optical material; however, the inherent brittleness hinders its further applications. Conventional toughening methods are realized based on the premise of a sacrifice in transparency and stiffness. In this work, we found an unprecedented strategy to address these obstacles by combining extensional stress-induced ductility and suppressing physical aging. PS-based film with a high stiffness, long-term ductility, and excellent transparency is achieved by introducing a styrene-butadiene block copolymer into the PS matrix and subsequently annealing stretched. A nanofibrillar structure of the polybutadiene (PB) phase is formulated surrounded by a PS matrix, and thus, the elongation at break enhances from 3.1% up to 86.8%, accompanying the yield strength enhanced from 25.5 to 62.2 MPa. More significantly, compared with neat PS, these films survive from physical aging and persistent ductility over time. The morphology deformation induced by stress makes an obvious contribution to the improvement of transparency. Investigating the dynamics of chain segments indicates that the incorporation of the copolymer can restrict rearrangement and local relaxation to the PS chain. This work could pave a potential route toward high-performance PS and might be transferable to other glassy polymers with a fragile character.

Graphene Oxide-Induced Interfacial Transcrystallization of Single-Fiber Milkweed/Polycaprolactone/Polyvinylchloride Composites

Understanding the interfacial crystallization is crucial for semi-crystalline polymer/natural fiber composites because it links to the final properties. This work reports, for the first time, the interfacial crystallization of a miscible blend between polycaprolactone (PCL) and polyvinylchloride (PVC) with milkweed fibers. We have first described the morphology of the fibers and the chemical composition of waxes covered on its surface. Our findings show that the transcrystallization (TC) layer of PCL/PVC could appear at the interface by simply coating with a layer of graphene oxide (GO) on the milkweed fiber. In our study, atomic force microscopy-infrared spectroscopy analysis shows that the crystallinity of the blends is higher at the vicinity of the interface compared to that in the bulk. The kinetic of the interfacial crystallization in terms of spherulite morphology and crystal growth rates at the nanoscale is examined. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy were used to analyze the prepared GO and evaluate its relationship with the interfacial crystallization behavior of the blends.

Polyethylene oxide enhances the ductility and toughness of polylactic acid: the role of mesophase

A lack of understanding of the structure-property relationship of the polylactic acid (PLA)-based polymer composite system makes it a challenge to manufacture products with optimized mechanical performance by precisely regulating the microscopic structure and morphology. Herein, we chose the PLA/polyethylene oxide (PEO) blend as a model to investigate the structural reason for the enhanced ductility and toughness of this kind of material. We have demonstrated that a considerable amount of the PLA mesophases exist in the melt quenched films that display high ductility and toughness, in contrast to the PLA crystals in their counterparts of slowly cooled films that are dominated by brittle fracture. The mesophase formed by melt quenching is attributed to a moderate acceleration of PLA chain mobility due to the plasticizing effect of the flexible PEO. In situ experiments have revealed the further formation of oriented mesophases induced by tensile deformation, which presents a high consistency between the content increase and the tensile stress intensification. We illustrate that the mesophases directly develop into a microfibrillar morphology to transmit the external stress and prevent crack propagation under deformation. This work emphasizes the essential role of the PLA mesophase in acquiring the enhanced ductility and toughness of the PLA/PEO composite films, which may be generalized to other similar PLA-based polymer composite materials.